In recent years UV-curable coatings have received wide attention as crosslinkable coatings because of their ease of application in the liquid state. During exposure to UV radiation the coating material crosslinks and becomes relatively impermeable to solvents, acids and water. For most applications the desired properties are toughness and high modulus. This is achieved by formulating an oligomeric material of molecular weight .about.1000 with a monomeric liquid in order to achieve the moderate viscosity needed for application to the part to be coated. Most typical commercial formulations contain an oligomeric material with two reactive sites and a monomeric material with one reactive site. See, for example, U.S. Pat. Nos. 4,099,837 and 4,125,644 and German Offenlegungsschrift No. 24 26 602. Both materials will undergo a chain reaction following initiation with a photoinitiator. The molecular weight between the reactive sites in the oligomeric material will thus determine the crosslink density, given that all reactive sites indeed undergo reaction. The monomeric monofunctional materials do not participate in the network formation and either homopolymerize or copolymerize without participating in the crosslinking. When homopolymerizing, they modify the properties of the cured film by acting similar to plasticizer. When copolymerizing, the monomer will extend the chains of the oligomer and incorporate its properties into the network.
Monomeric materials with three or four reactive sites are often used in addition to or instead of the monofunctional material, to increase the crosslink density.
In fiber optic applications such as single buffer coatings and/or as bonding materials for fiber packaging, low modulus materials are desired with sufficient toughness to withstand abrasion and also to minimize microbending induced transmission losses. Also environmental cycling requirements from -40 degrees C. to 90 degrees C., and the need to minimize mechanical deformations as induced by temperature dependent properties make it desirable that fiber optic materials do not have any transitions within the above temperature range. Given that their modulus should remain relatively low, a material is desired which will remain in the rubbery state throughout the above temperature range.
In commercial formulations, materials that cure to form soft crosslinked polymers are obtained by increasing the amount of monomeric diluent and/or by using elastomeric oligomers such as polyurethanes preferentially to, for example, polyesters.
The lower modulus is achieved by plasticization with a homopolymer having in general a low glass transition temperature. A disadvantage of this method is that material properties are determined by the amount of monomeric diluent, and small variations in the composition usually imply relatively large changes in mechanical properties. The latter is avoided by the approach that we have taken in order to obtain soft UV-radiation crosslinkable materials, but with sufficient toughness to withstand abrasion.
We establish five desirable criteria for the design and formulation of soft UV-radiation crosslinkable materials:
1. The materials used in the prepolymer should be primarily elastomeric oligomers rather than monomeric compounds with high entropy functional groups: examples for these oligomers are O-aliphatic, N-aliphatic urethanes and aliphatic polyethers. These oligomers have low temperature glass transitions. Crosslinked polyetheracrylates and methacrylates also show reduced water permeability and absorptivity compared to more polar oligomers such as polyesters and urethane urea oligomers. PA1 2. Another manner of obtaining crosslinked soft rubbery materials with low temperature glass transitions is the use of materials with the potential of lowering the symmetry of the network, such as would be the result of using branched oligomers and mixtures of acrylate and methacrylate terminated oligomers. The latter have a high entropy and tend not to aggregate even at low temperatures. PA1 3. In order to obtain a homogeneous network, the mechanical and physical properties of which will be less dependent on the exact formulation, all oligomers should be able to take part in the crosslinking reaction. This is achieved by using only materials with two or more reactive groups. The crosslink density can now be achieved by adjusting the molecular weight between reactive groups. PA1 4. Also in order to obtain a homogeneous network, materials with similar polymerization rate constants should be used. As a result of this, copolymerization, rather than homopolymerization, is expected to occur. PA1 5. Strong intermolecular interactions should be avoided, since they create a strong temperature dependence of the physical and mechanical properties. Molecules with aliphatic and non-polar groups rather than aromatic and/or polar groups fulfill this requirement.